Abstract
A low-inductance drive coil energized with a shaped high-current drive pulse, together with a separate pickup coil, is used for crack detection in thick multiple-layer aircraft structures. The slow rise time or low-frequency portion of the shaped pulse applied to a solenoid drive coil is used to obtain the required depth-of-field penetration into the structure. The fast fall time or high-frequency portion of the shaped pulse is used to rapidly cancel the slow pulse current and field at the structure surface. This cancellation process causes the collapse of the field within the structure, which reacts with the structure and cracks. The structure/crack eddy-current response is detected at the structure surface with a spiral pancake pickup coil. The pancake pickup coil is used to selectively detect the tangential component of the drive pulse field collapsing from within the structure. An important aspect of this high peak energy testing approach is its saturation of steel fasteners, which highly reduces their permeability variations.
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